Quieting of low-frequency vessel noise in Monterey Bay National Marine Sanctuary during the COVID-19 pandemic

Low-frequency sound from large vessels is a major source of ocean noise, overlapping the range of marine animal communication. Changes in vessel activity provide opportunities to quantify the relationship between traffic levels and soundscape conditions in biologically important habitats. Using continuous deep-sea (890 m) recordings ∼20 km from shipping lanes, we observed quieting of low-frequency noise within Monterey Bay National Marine Sanctuary (California, USA) during the COVID-19 pandemic. Noise levels in the frequency band 31–100 Hz, which captured large vessel noise while minimizing potential biases from geological and biological sound sources, decreased during January–June 2020. At monthly resolution the 2020 quieting trend correlated with decreasing cumulative hours of vessel presence derived from Automatic Identification System (AIS) data for cargo vessels (r = 0.92; p = .01), and the sum of all positively correlated vessel types (cargo, towing, dredging, passenger, tanker; r = 0.88; p = 0.02). February–June 2020 levels were up to 2.7 dB re 1 μPa2/Hz below baseline of the two years prior. While anticipated to be short-term, at their peak these changes represent nearly a halving of acoustic power, emphasizing the significance of offshore large vessel traffic to sound levels in deeper waters in the sanctuary.